#include <stdio.h>
#include <stdlib.h>


/* Caps out at n = 20 before it overflows. */
unsigned long long int
factorial(unsigned long long int n)
{
    if (n == 1)
	return 1;
    return n * factorial(n-1);
}


int
main(int argc, char *argv[])
{
    printf("%lld\n", factorial(atoi(argv[1])));

    return 0;
}
